1. Field of the Invention
The present invention relates to an ink jet printing method and an ink jet printing apparatus in which an ink is ejected to form an image on a printing medium. More specifically, this invention relates to an ink jet printing method and an ink jet printing apparatus in which a liquid is ejected to insolubilize or coagulate a coloring material in the ink.
2. Description of the Related Art
Heretofore, an ink jet printing apparatus for making printing onto a printing medium such as paper, cloth, plastic sheet, OHP sheet, and the like, since it is possible to make high-density and high-speed printing, has been utilized and commercialized as output means of an information processing system, for example, a printer as an output terminal of a copier, a facsimile, an electronic typewriter, a word processor, a workstation, and the like, or a handy or portable printer for a personal computer, a host computer, an optical disk apparatus, a video apparatus, and the like.
In this case, the ink jet printing apparatus has a construction for meeting the function and application mode specific to the apparatus. In general, an ink jet printing apparatus comprises a carriage including printing means (printing head) and an ink tank, a transportation means for transporting the printing paper, and a control means for controlling these components. During printing time, the printing head for ejecting ink droplets from a plurality of ejection openings is serially scanned in a direction (main-scanning direction) perpendicular to the transportation direction (sub-scanning direction) of the printing medium, whereas during non-printing time, the printing medium is intermittently transported in an amount equal to the printing width. This method is to make printing by ejecting an ink onto the printing medium according to a printing signal, and is widely used as a low running cost and quiet printing method. Further, by using the printing head having a plurality of nozzles for ejecting ink which are arranged on a straight line in the sub-scanning direction, the printing head is scanned on the printing medium to make printing having a width corresponding to the number of the nozzles. Then, a high-speed printing operation can be achieved.
Further, recently, the ink jet printing apparatus is practically used, which is equipped with 3 to 4 colors of the printing heads to enable formation of an image in full color. This apparatus can be equipped with three types of printing heads corresponding to the three primary colors of yellow (Y), magenta (M), and cyan (C) or four types of printing heads corresponding to these three primary colors and black (B).
However, because, in the conventional ink jet printing method and apparatus, prevention of ink bleeding occurring between individual colors of black (B), yellow (Y), magenta (M), and cyan (C), and increase in density of black image and prevention of feathering are contradictory problems, it is difficult to achieve the printing quality of color printing to a level sufficient for meeting the user needs. The reason will be described below.
In general, when a color image is formed on a plain paper by the ink jet printing method, a quick-drying ink which is fast in penetration speed into the plain paper is used. Therefore, ink bleeding can be prevented in a boundary area between individual colors constituting the image. However, when the quick-drying ink is used, the black image portion tends to be low in density and the colored image portion other than black tends to be low in color formation density. Further, when a line image represented by letters is printed, the ink tends to bleed along fibers of the paper. This results in a so-called feathering. In particular, letters printed by a black ink tend to have remarkable feathering as compared with other colors, resulting in unclear letters of so-called less sharpness. As a result, the quality of the printed image is considerably deteriorated as a whole.
Generally, to obtain a high quality image which is high in density of the black image portion and free of feathering, it is necessary that an ink relatively low in penetration speed onto the plain paper is used and is ejected in a large amount to some extent. However, in this case, the black ink and color inks bleed in the adjacent boundary area of the black image portion with the color image portions, thereby considerably degrading the quality of the printed image.
To improve these defects, a method is practically used in which a heater is provided in the printing apparatus for promoting drying of the inks, thereby obtaining a color image of high color formation and without bleeding between colors. However, it is clear that this method cannot be avoidable in a size increase of the apparatus and a cost increase.
As described above, bleeding prevention of inks between black and individual colors, high-density of black image, and prevention of feathering are contrary problems to each other.
Then, Japanese Patent Application Laid-open No. 3-146355 proposes a method in which an area along the boundary area between black and colors is not printed. However, this method has a problem in that the printed data is changed.
Further, Japanese Patent Application Laid-open No. 4-158049 proposes a method which has heads of a plurality of colors for color printing and a head for letter printing, the plurality of color printing heads and the letter printing head being selected according to the image to be printed. In this method, when a black image printed by the color printing heads and a black image printed by the letter printing head are mixed, a sense of incompatibility occurs due to a difference in quality between both.
Still further, there is considered a method in which the black area along the boundary area between black and color is printed by ejecting the color inks in overlapping manner, thereby preventing bleeding in the boundary area between black and color. Although, in principle, black is obtained by overwriting (mixing) three colors of Y, M, and C, the black image formed by mixing color inks in this method is inferior in color formation as compared with ordinary black ink.
On the other hand, Japanese Patent Application Laid-open No. 56-84992 and Japanese Patent Application Laid-open No. 64-63185 disclose a technology using a liquid for insolubilizing a dyestuff in the ink.
Japanese Patent Application Laid-open No. 56-84992 discloses a method in which the printing paper is previously applied with a material for fixing the dyestuff. However, this method has problems to be solved in that it is required to use a specific printing paper, and for the application of the material for fixing the dyestuff, an increase in apparatus size and a cost increase are unavoidable, and it is difficult to apply the above material on the printing paper stably to a predetermined film thickness.
Yet further, Japanese Patent Application Laid-open No. 64-63185 discloses a technology for depositing a colorless ink for insolubilizing the dyestuff onto the printing paper by an ink jet printing head. With this method, since the dot diameter of the colorless ink is set greater than the dot diameter of the imaging ink, predetermined characteristics can be satisfied even when the application positions of an imaging ink and the colorless ink are deviated from each other. In this method, since the amount of the colorless ink applied to the portion corresponding to the image position is larger than usual, there is a problem to be solved in that not only the ink drying time is increased, but also a very unclear image is resulted.
Yet further, Japanese Patent Application Laid-open No. 7-195823 described that the printing paper surface is applied with the above colorless substance prior to ink jet printing, thereby particularly enabling color printing by one pass.
As described above, the methods disclosed in the prior art have problems to be solved.
By the way, as described above, when the ink and a printing ability improving liquid for insolubilizing or coagulating the coloring material in the ink are in contact with each other on the ejection opening face (or a face) to react on each other, an adhesion occurs on the ejection opening face, which results in deflecting of ink droplets leading to image degradation and results in ejection missing due to clogging of the ejection openings, thus greatly affecting the reliability.
One of the causes is rebounding of the ink or the printing ability improving liquid from the paper surface when they are ejected thereto.
The inventors have found that, with respect to generation of rebounding droplets from the paper surface, an amount of rebounding is also changed according to a print duty of the image. The droplets due to rebounding are small in amount when the print duty is low, whereas the droplets due to rebounding are large in amount when the print duty is high, thereby affecting the reliability of printing.